1. Field of the Invention
The present invention relates to a method and apparatus for simultaneously performing frequency resource sensing and data transmission in a wireless communication system.
2. Discussion of the Related Art
Cognitive Radio (CR) was introduced to efficiently use frequency resources in an environment in which available frequency resources have been reduced in view of the rapid development of wireless communication service and already allocated frequency resources have been used less frequently. With the CR technology, a wireless device autonomously detects and uses unused frequency resources, thus conducting efficient communication without frequency resource waste.
A Secondary User (SU) can search for frequency resources unused by a Primary User (PU) through spectrum sensing and thus efficiently use the detected frequency resources. In the CR-spectrum sensing environment, the SU should be able to determine the presence or absence of a signal from a PU transmitter with a target detection probability Pd set by a PU system. If the SU determines the presence of a signal from the PU transmitter in a specific frequency band, the SU does not use the frequency band. On the other hand, if the SU determines the absence of a signal from the PU transmitter end in a specific frequency band, the SU may use the frequency band. In this CR-spectrum sensing environment, a surrounding environment may be sensed largely by quiet sensing or active sensing.
Quiet sensing is a spectrum sensing technique in which an SU first determines whether a PU is using a channel of a specific frequency band, before transmission and then only when the channel is empty, the SU transmits data in the specific frequency band. The transmitter of the SU performs quiet sensing according to a frame structure illustrated in FIG. 1. If one transmission frame includes N subframes, the SU transmitter senses whether an intended frequency band is available prior to data transmission by quiet sensing. Specifically, the SU transmitter senses a PU signal during a time period corresponding to Nq subframes, to thereby determine the presence or absence of the PU signal. A PU detection probability is calculated by the following equation using an energy detector.
                                          P            d                    ⁡                      (                                          N                q                            ,              M                        )                          =                  Q          [                                    η              -                                                MN                  q                                ⁡                                  (                                                            SNR                      P                                        +                    1                                    )                                                                                                                          N                    q                                    ⁢                                                            ∑                                              i                        =                        1                                            M                                        ⁢                                                                  (                                                                                                            SNR                              p                                                        ⁢                                                          λ                              i                                                                                +                          1                                                )                                            2                                                                      +                                                      SNR                    P                                    ⁢                                                            MN                      q                                                                                                    ]                                    [                  Equation          ⁢                                          ⁢          1                ]            
In [Equation 1], Q( ) is a q-function, M is the number of antennas, SNRP is the Signal-to-Noise Ratio (SNR) of a signal received from a PU transmitter and measured at the SU transmitter, η is a threshold level needed to determine the presence or absence of a PU to satisfy a target detection probability PD at the energy detector, expressed as
      η    =                                        Q                          -              1                                ⁡                      (                                          P                _                            D                        )                          ⁢                                            N              q                        ⁢                                          ∑                                  i                  =                  1                                M                            ⁢                                                (                                                                                    SNR                        p                                            ⁢                                              λ                        i                                                              +                    1                                    )                                2                                                        +                        MN          q                ⁡                  (                                    SNR              p                        +            1                    )                      ,and λi is an ith eigenvalue of a channel between the PU transmitter and the SU transmitter. That is, the number of samples Nq for sensing is determined according to the target detection probability Pd and the surrounding environment represented by the SNR SNRP. If the SU transmitter determines that the frequency band is available, the SU transmitter transmits data during the remaining time (i.e. N−Nq subframes). That is, when a false alarm is not generated with the target detection probability satisfied, the SU transmitter transmits data.
A normalized throughput of quiet sensing, TM,Q is given by
                              T                      M            ,            Q                          =                              {                          1              -                                                P                                      f                    ,                    q                                                  ⁡                                  (                                      M                    ,                                          N                      q                                                        )                                                      }                    ⁢                      (                          1              -                                                N                  q                                N                                      )                    ⁢                      log            2                    ⁢          det          ⁢                                                                I                                  M                  R                                            +                                                                    SNR                    S                                    M                                ⁢                                  R                  t                                      1                    /                    2                                                  ⁢                                  HR                  r                                ⁢                                  H                  H                                ⁢                                  R                  t                                      H                    /                    2                                                                                                                      [                  Equation          ⁢                                          ⁢          2                ]                                                      P                          f              ,              q                                ⁡                      (                          M              ,                              N                q                                      )                          =                  Q          ⁡                      [                                                                                Q                                          -                      1                                                        ⁡                                      (                                                                  P                        ~                                            d                                        )                                                  ⁢                                                                            1                      /                      M                                        ⁢                                                                  ∑                                                  i                          =                          1                                                M                                            ⁢                                                                        (                                                                                                                    SNR                                p                                                            ⁢                                                              λ                                l                                                                                      +                            1                                                    )                                                2                                                                                                        +                                                SNR                  p                                ⁢                                                      MN                    q                                                                        ]                                              [                  Equation          ⁢                                          ⁢          3                ]            
where Pf,q(M,Nq) is a false alarm probability, 1−Pf,q(M,Nq) is a non-false alarm probability, that is, a transmittable probability, and (N−Nq)/N is a ratio of a data transmittable time period. IMR is an MR×MR unit matrix, H is an Nr×Nt channel matrix, and Rt and Rr are Nt×Nt and Nr×Nr antenna correlation matrices, respectively. det( ) is the determinant of a matrix.
Active sensing is a technique in which time resources are not divided for sensing and a transmitting SU terminal continues data transmission, performing PU sensing with the aid of an inactive terminal (i.e. a terminal that is not transmitting or receiving data) within the same network. FIG. 2 illustrates a system model and frame structure for active sensing. Referring to FIG. 2, compared to quiet sensing, the inactive SU performs PU sensing during all N subframes and the transmitting SU transmits data also in all of the N subframes. Active sensing is characterized in that a transmission entity and a sensing entity are different. A normalized throughput of active sensing, TM,A is given by
                              T                      M            ,            A                          =                              {                          1              -                                                P                                      f                    ,                    a                                                  ⁡                                  (                                      M                    ,                    N                                    )                                                      }                    ⁢                      log            2                    ⁢          det          ⁢                                                                I                                  M                  R                                            +                                                                    SNR                    S                                    M                                ⁢                                  R                  t                                      1                    /                    2                                                  ⁢                                  HR                  r                                ⁢                                  H                  H                                ⁢                                  R                  t                                      H                    /                    2                                                                                                                      [                  Equation          ⁢                                          ⁢          4                ]                                                      P                          f              ,              q                                ⁡                      (                          M              ,              N                        )                          =                  Q          ⁡                      [                                                                                Q                                          -                      1                                                        ⁡                                      (                                                                  P                        ~                                            d                                        )                                                  ⁢                                                                            1                      /                      M                                        ⁢                                                                  ∑                                                  i                          =                          1                                                M                                            ⁢                                                                        (                                                                                                                                                      SNR                                  p                                                                /                                                                  (                                                                                                            SNR                                      s                                                                        +                                    1                                                                    )                                                                                            ⁢                                                              λ                                l                                                                                      +                            1                                                    )                                                2                                                                                                        +                                                                    SNR                    p                                    /                                      (                                                                  SNR                        s                                            +                      1                                        )                                                  ⁢                                  MN                                                      ]                                              [                  Equation          ⁢                                          ⁢          5                ]            
Herein, SNRs is the SNR of a signal received from the transmitting SU and measured at the inactive SU. Compared to quiet sensing, active sensing offers the benefit of no time loss for data transmission because data is transmitted in all N subframe. From the perspective of false alarm performance, it may be said that interference from a signal transmitted by the transmitting SU is a loss and the increase of sensing samples from Nq to N is a gain.
It is important to determine the lowest limit of the SNR SNRP at which a PU signal is supposed to be detected in the above-described CR-spectrum sensing environment. In this regard, a Federal Communications Commission (FCC) document says that a sensing-based device should be able to detect a Digital TV (DTV) signal of −114 dBm and a low-power auxiliary signal of −107 dBm. Given an average TV-band noise power of −98 dBm, an SU should be able to sense at least a DTV signal with an SNRP value of −16 dB or above and at least a low-power auxiliary signal with an SNRP value of −9 dB or above.
Bi-directional communication means that one node performs both transmission and reception. If there are a pair of nodes that want to transmit and receive signals, the nodes can exchange signals at the same time without a transmission and reception delay by bi-directional communication. Conventionally, simultaneous transmission and reception in one node is not easy due to hardware limitations. When a node receives a signal from the other party simultaneously with signal transmission, the transmitted signal of the node serves as self-interference to the node. However, the past few years has witnessed suggestion and implementation of self-interference cancellation techniques in terms of hardware and software. The self-interference cancellation techniques have been actively studied, which use a precoder, beamforming, a reception filter, etc. based on transmission signal information and channel information about a transmitter itself. A device that can perfectly eliminate an echo signal whose strength is different from those of a transmission signal and a reception signal by 70 dB to 90 dB can be achieved in hardware based on isolation in order to conduct bi-directional communication.
In the case where an energy detector is used, if the surrounding environment changes and as a result, an actual value of SNRP is different from a known value of SNRP, the sensing performance of the energy detector that detects the presence or absence of a PU signal is degraded. In this context, blind sensing algorithms have been proposed, which perform sensing using multiple antennas without oversampling a received signal or using the SNR of a received signal, SNRP. In these algorithms, statistical signal characteristics are calculated using linear prediction, QR decomposition, and oversampling of a received signal, with respect to the presence of a PU signal and the absence of a PU signal. Because an oversampled signal having a signal and noise mixed in it has a high correlation, relative to an oversampled signal including noise only, the capability of detecting a PU signal even at a low SNRP can be increased significantly by the use of a correlation-based detection ratio.
However, in the case of quiet sensing, a sensing period (Nq subframes) is needed for accurate sensing. The resulting decrease of a transmission period (N−Nq subframes) leads to a lower throughput.
Active sensing faces the following problems. First of all, since active sensing needs the aid of another inactive SU, sensing power is additionally consumed. Due to the need for feedback of sensing information, power for transmission and radio channel resources for feedback are additionally used. In addition, a transmission entity and a sensing entity are located at different positions. Consequently, feedback is delayed and it is difficult to ensure the accuracy of sensing information. Finally, although the reliability of a link on which a sensing result is transmitted increases with the link quality between the transmitting SU and the inactive SU, a signal transmitted from the transmitting SU serves as interference, thus decreasing the accuracy of sensing information. On the other hand, as the link quality gets poorer, less interference is caused and thus sensing information is more accurate. However, the reliability of the feedback link is decreased.
To solve the foregoing problems, the transmission entity and the sensing identity should be identical and a method of dividing resources other than time resources is needed to allow simultaneous transmission and sensing.
A heterogeneous network refers to a communication network in which homogeneous or heterogeneous systems are deployed in the same area and thus operate with mutual influences. Examples of such a heterogeneous network include a Hierarchical Cell Structure (HCS), a femtocell, a smart grid network, etc.
For active transmission in the above-described environment, co-existence management of neighboring networks is essential. Sharing limited frequency resources between systems causes problems. The channel quality of an intended system to be used as well as the channel quality of an existing system is degraded. To avert this problem, it is necessary to move to another channel if it is determined by sensing the states of neighbor channels that the quality of a current channel gets poor or to select another channel if an intended channel has a poor quality.
A PU and a plurality of SUs co-exist in a CR-heterogeneous network environment. To sense a surrounding situation and select a channel to be used for the purpose of co-existence between SUs in time resources, a centralized control scheme and a decentralized control scheme are largely available.
According to the centralized control scheme, co-existence between SUs is managed through a central controller or a DataBase (DB). Specifically, after the DB or the controller collects all information including information about the position or channel use or non-use of each user, it records channel use information about each SU or allocates a requested channel to an SU.
Each SU acquires information about channels available at its location from a DB of a PU or sensing information about a PU signal in the centralized control scheme. Subsequently, the SU determines whether an intended channel is being used by another SU. If the channel is empty, the SU uses the channel and updates information about the channel in an SU database. On the contrary, if the intended channel is being used by another SU, the SU determines whether any other channel is available. If all channels available to the SU at its current location are occupied by other SUs, the SU waits or selects a channel that causes least interference currently, uses the selected channel, and registers the channel in the SU DB.
According to the decentralized control scheme, individual nodes share frequency resources, cooperating with one another in an interference avoidance mechanism, rather than a specific controller decides for each node whether the node is supposed to use a channel. Each SU is synchronized with other SUs and determines whether the other SUs are using channels or interfering by transmitting a beacon signal in time synchronization. Since a specific controller does not decide whether the SU is to use a channel, the individual SU decides whether to use a channel according to its own judgment under a condition set by the network.
In the decentralized control scheme, each user acquires information about channels available at its location from a PU DB or by sensing a PU signal and then monitors a beacon signal during a co-existence window period. In the absence of a beacon signal on an intended channel to be used, which means that no neighboring SU is using the channel, the SU determines to use the channel and transmits a beacon signal during the next co-existence window period. On the contrary, in the presence of a beacon signal on the intended channel to be used, the SU checks whether a beacon signal exists on any other channel and then uses a channel free of a beacon signal. If all channels available to the SU at the current location have beacon signals, the SU waits until the next co-existence window or selects a channel that causes least interference at the moment, uses the selected channel, and transmits a beacon signal during the next co-existence window period.
If the length of one frame is M, the length of available time resources is Mc, the number of antennas at an SU transmitter is Nt, and the number of antennas at a receiver is Nr, the average SU throughput Tc of the conventional technique that divides time resources is calculated by [Equation 6].
                              T          c                =                              max                          i              ∈              A                                ⁢                                    (                              1                -                                                      M                    c                                    M                                            )                        ⁢                          E              ⁡                              [                                                      log                    2                                    ⁢                  det                  ⁢                                                                                                        I                                                  N                          r                                                                    +                                                                                                    SNR                            s                                                                                N                            t                                                                          ⁢                                                  R                          t                                                      1                            /                            2                                                                          ⁢                                                  HR                          r                                                ⁢                                                  H                          H                                                ⁢                                                  R                          t                                                      H                            /                            2                                                                                                                                                                  ]                                                                        [                  Equation          ⁢                                          ⁢          6                ]            
Herein, A is a set of available channels, E( ) is an expectation function representing an average, det(K) is a determinant of matrix K, INr is an Nr×Nr unit matrix, SNRs is an SNR of a secondary link, H is an Nr×Nt channel matrix, and Rt and Rr are Nt×Nt and Nr×Nr antenna correlation matrices, respectively.
A Macro-cell Base Station (MBS) is overlaid with a plurality of Small-cell Base Stations (SBSs) in a heterogeneous cellular network environment. The size of the MSB is different from the sizes of the SBSs. In the heterogeneous cellular network environment, time resources and frequency resources are allocated in the centralized or decentralized control scheme as in the CR-heterogeneous network environment. An MBS allocates time/frequency resources to each SBS by scheduling them in the centralized control scheme, whereas an individual SBS determines a surrounding situation on its own and uses a frequency based on the determination in the decentralized control scheme.
In the heterogeneous cellular network environment, each SBS measures the interferences of available frequency resources and selects a least interfering channel based on the interference measurements. Once a frequency band (i.e. a channel) is selected, the SBS transmits data on the selected channel until it ends the data transmission. When a data transmission frame ends, the SBS selects a frequency band for the next frame based on interference measurements.
The centralized control scheme requires an additional DB to collect information, thus incurring cost to build and manage the DB. Moreover, a data transmission time is reduced by as much as a time taken to retrieve necessary information from the DB or a time taken to register information in the DB. As a result, a throughput is decreased. In contrast, the decentralized control scheme is relieved of the cost of building and managing an additional DB because there is no need for the additional DB to collect information. However, due to transmission and sensing of a beacon signal during a co-existence window period, a data transmission is decreased by as much as the co-existence window period, thereby decreasing a throughput. Both of the above-described schemes take much time to select a channel. The resulting decrease of a time period assigned to data transmission leads to a lower throughput.